1. Field of the Invention
The invention relates to printer inks and printing systems.
2. Related Art
Viscosity for an ink is a measure of the ink's thickness. Low viscosity printer ink loses shear strength at high temperatures even when disposed on a carrier such as a printer ink ribbon. Within impact printing applications such as those using an ink ribbon, this can result in ink smearing and ink migration. This lowers the print quality.
On the other hand, the viscosity of an ink that performs well at elevated temperatures becomes excessively high as to its viscosity at lower temperatures. Excessively high ink viscosity exhibits other printing problems. The problems can include poor transfer into and out of the printer ribbon, resistance to pumping through small tubing, and a very slow transfer through foam materials. Such foam materials can be used in an ink reservoir roller to replace ink within the printer ribbon.
An ideal printer ink should flow easily when the ambient temperature is cold. The ideal ink should also remain thick enough so that it will not excessively migrate when the temperature is hot. Low ambient temperatures require a light (i.e., low viscosity) ink and high temperature requires a heavy (i.e., high viscosity) ink.
Viscous flow as to ink can be pictured as taking place by the movement of molecules or segments of molecules from one place in a lattice to a vacant hole. The total “hole” concentration can be regarded as a space free of polymer or free volume (see, e.g., Rodriguez, F., Principles of Polymer Systems, 3:177, 1989). Doolittle proposed (Doolittle, A. K., J. Appl. Phys., 22:1471, 1951) that the viscosity should vary with the free volume and free volume is expected to vary with temperature. The diffusion and movement is closely related to the size of a molecule represented by the hydrodynamic volume.
Low temperatures are favorable to small molecule movement whereas high temperatures are conducive to the movement of either small or large molecules. Thus when inks having small molecules are exposed to high temperatures, they move with great freedom. Inks having large molecules can also move freely at high temperatures but not as freely as with small molecules, and not effectively at low temperatures.